Pump with variable stroke piston

ABSTRACT

A pump apparatus includes a housing located on an axis. The housing has a chamber, an inlet valve and an outlet valve. A piston driver is configured to axially reciprocate. A piston is a piston configured to reciprocate in the chamber to draw liquid into the chamber through the inlet valve during an intake stroke and to discharge the liquid out of the chamber through the outlet valve during a delivery stroke. A spring axially biases the piston to a base position relative to the driver, so that the driver, when reciprocating, will drive the piston to reciprocate. A preload structure preloads the spring to enable pressure of the liquid in the chamber to displace the piston away from the base position against the spring bias after the pressure exceeds a threshold level.

TECHNICAL FIELD

This application relates to liquid pumps.

BACKGROUND

A liquid pump includes a piston that reciprocates in a cylindricalchamber. The piston draws liquid through an inlet valve into the chamberduring an intake stroke and forces the liquid out of the chamber throughan outlet valve during a delivery stroke.

SUMMARY

A pump apparatus includes a housing located on an axis. The housing hasa chamber, an inlet valve and an outlet valve. A piston driver isconfigured to axially reciprocate. A piston is a piston configured toreciprocate in the chamber to draw liquid into the chamber through theinlet valve during an intake stroke and to discharge the liquid out ofthe chamber through the outlet valve during a delivery stroke. A springaxially biases the piston to a base position relative to the driver, sothat the driver, when reciprocating, will drive the piston toreciprocate. A preload structure preloads the spring to enable pressureof the liquid in the chamber to displace the piston away from the baseposition against the spring bias after the pressure exceeds a thresholdlevel.

Preferably, the spring has a spring constant that increases withincreasing compression of the spring. The spring is configured to renderthe volume of liquid delivered during each delivery stroke inverselyrelated to output pressure of the pump. The preload is manuallyadjustable. The preload structure includes a protrusion on the pistonwithin the driver, and further includes a stop surface in the driverthat blocks the protrusion from exiting the driver and against which theprotrusion is biased by the spring. The spring is configured to absorbthe entire reciprocation of the driver in a situation where liquid isblocked from exiting the outlet valve piston while the driver continuesto reciprocate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a pressure washer that includes a pump;

FIGS. 2-4 are schematic sectional views of the pump at different stagesduring its operation; and

FIG. 5 is a schematic view of a spring of the pump.

DESCRIPTION

The apparatus 1 shown in FIG. 1 has parts that are examples of theelements recited in the claims. The apparatus thus includes examples ofhow a person of ordinary skill in the art can make and use the claimedinvention. It is described here to meet the requirements of enablementand best mode without imposing limitations that are not recited in theclaims.

The apparatus 1 is a pressure washer. It includes a pump 10 for pumpinga liquid from a supply line 12 to an outlet line 14. The supply line 12has an inlet hose 20 with a threaded end 22 configured to be screwedonto a water faucet. The outlet line 14 has an outlet hose 24 connectedto a spray nozzle 26. The pump 10 draws water from the inlet line 12 andforces it out the nozzle 26 in the form of a pressurized spray.

As shown in FIG. 2, the pump 10 includes a housing 30 located on acentral axis A. The housing 30 has axially front and rear ends 32 and 34and a cylindrical piston-bearing surface 36 defining a cylindricalchamber 38. The chamber 38 is centered on the axis A and extends forwardfrom a rear opening 40 of the housing 30. Liquid enters the chamber 38from the supply line 12 through an inlet check valve 42. The liquidexits the chamber 38 into the outlet line 14 through an outlet checkvalve 44.

A piston 50 includes piston head 52 rigidly fixed to a piston rod 54. Athreaded front end 56 of the rod 54 is screwed into a threaded bore 57of the head 52. The length L of the piston 50 depends on the depth towhich the rod 54 is screwed into the head 52. The head 52 extends fromthe rod 54 into the chamber 38. It forms an annular liquid-tight sealwith, and is axially slidable against, the piston-bearing surface 36.The head 52 and the housing 30 together enclose a compression cavity 58,which is a closed section of the chamber 38 that has a volume thatvaries as the head 52 reciprocates. A nut 60 is screwed onto the rearend 62 of the rod 54 and protrudes radially outward from the rod 54.

The rear end 62 of the rod 54 is captured in a bore 70 of a pistondriver 72. A threaded ring 74 surrounding the rod 54 is screwed into athreaded front end 76 of the bore 54. A rearward-facing stop surface 78of the ring 74 blocks the nut 60 from exiting the bore 70.

A bias spring 80 is wrapped about the rod 54 and compressed betweenrespective spring bearing surfaces 82 and 84 of the head 52 and thedriver 72. The spring 80 biases the rod 54 into a base position relativeto the driver 72, as shown in FIG. 2, in which the nut 60 abuts the stopsurface 78. The nut 60 and the stop surface 78 thus together preload thespring 80. The stop surface 78 is axially between the nut 60 and thebias spring 80.

A return spring 90 is wrapped about the piston head 54 and compressedbetween respective spring bearing surfaces 92 and 94 of the housing 30and the head 52. The return spring 90 keeps the driver 72 in contactwith a front wobble surface 96 of a wobble plate 98. The plate 98 isattached to an axially-extending output shaft 100 of a motor 102. Thewobble surface 96 is inclined with respect to the axis A so that itreciprocatingly pushes the driver 72 forward against the bias of thereturn spring 90 as the plate 98 rotates. The piston 50 is driven by thedriver 72 to reciprocate, with a series of intake and delivery strokesin phase with forward and rearward strokes of the driver 72.

The delivery stroke starts with the piston 50 fully retracted as shownin FIG. 2, and pressure P_(cav) in the cavity 58 equaling supply linepressure P_(in) plus crack pressure P_(crack) of the inlet valve 42.Thereafter during the delivery stroke, the piston 50 advances, causingthe pressure in the cavity 58 to increase. At some point, as in FIG. 3,when the cavity pressure P_(cav) starts to exceed P_(out)+P_(crack), theoutlet valve 44 starts to open to let the liquid into the outlet line14. From then on, further advancement of the piston 50 delivers liquidinto the outlet line 14 while P_(cav) remains constant atP_(out)+P_(crack). This continues until the piston 50 reaches a fullyforward position shown in FIG. 4, the outlet valve 44 closes, and cavitypressure P_(cav) remains at P_(out)+_(crack).

The intake stroke starts with the piston 50 fully extended as shown inFIG. 4. As the return spring 90 pushes the piston 50 rearward, cavitypressure P_(cav) gradually decreases. When P_(cav) recedes belowP_(in)−P_(crack), the inlet valve 42 opens to let liquid from the supplyline 12 into the cavity 58. Further retraction of the piston 50 drawsliquid through the inlet valve 42 into the cavity 58, while P_(cav)remains constant at P_(in)−P_(crack). The intake stroke ends as shown inFIG. 2 with the piston 50 fully retracted.

During the delivery and intake strokes, the bias spring 80 functions asfollows: At the start of the delivery stroke, portrayed in FIG. 2, thecavity pressure P_(cav) is too weak to overcome the preload of the biasspring 80 urging the nut 60 against the stop surface 78. At some pointduring the delivery stroke, if and when the cavity pressure P_(cav)increases sufficiently to overcome the preload, the nut 60 will start toseparate from the stop surface 78. For comparison purposes, FIG. 4 showsthe positions of the driver 72 and the piston 50 at the start of thedelivery stroke in dashed lines and their positions at the end of thedelivery stroke in solid lines. By the end of the delivery stroke, thedisplacement distance D_(p) of the piston 50 is shorter than thedisplacement distance D_(D) of the driver 72 by the separation distanceAD of the nut 60 from the stop surface 78.

If the output pressure P_(out) remains below a threshold levelsufficient to overcome the spring preload, ΔD will be zero. Above thatthreshold, over a range of output pressures P_(out) for which the pump10 is designed, ΔD is a smooth positive function of output pressureP_(out). The function is “positive” in that ΔD increases with increasingP_(out) throughout the pressure range, and “smooth” in that the secondderivative of ΔD verses P_(out) is finite over the operating range. Dueto the density and incompressibility of the liquid filling the cavity58, ΔD is substantially unaffected by inertia of the piston head 52.

The delivery stroke volume, i.e., the volume of liquid delivered duringeach delivery stroke, is proportional to the displacement D_(P) of thepiston 50, which equals displacement D_(D) of the driver 72 minus ΔD.Therefore, when P_(out) is above the threshold pressure, the deliverystroke volume is smoothly and inversely related to P_(out). When P_(out)is below the threshold pressure, the delivery stroke volume isunaffected by varying P_(out).

The threshold can be manually increased by increasing the preload on thebias spring 80. This can be done by screwing the rod 54 deeper into thehead 52 or screwing the ring 74 deeper into the driver 72. Either ofthese steps decreases the depth of the head 52 in the chamber 38. Theresulting increase in initial volume of the cavity 58 does not affectthe achievable output pressure Pout, because the liquid isincompressible.

Power input by the pump 10 from the motor 102 is typically proportionalto motor speed, delivery stroke volume and outlet pressure P_(out).Since the delivery stroke volume of this pump 10 decreases withincreasing P_(out), the required power will tend to vary less withP_(out) than without the reduction ΔD in stroke displacement.

Preferably, the bias spring 80 is selected to yield a delivery strokevolume that is approximately inversely proportional to P_(out), i.e.,proportional to 1/P_(out). That renders the input power approximatelyinvariant with P_(out), so that a motor 102 optimized for one powerlevel at one outlet pressure would be optimal for other pressures too.This can be achieved by the bias spring 80 having a spring constant thatincreases with increasing spring compression. A step-wise increasingspring constant can be achieved by the bias spring 80 comprising coilsprings 111 and 112 differing in spring constant. In the example shownin FIG. 5, the springs 111 and 112 are arranged in parallel, morespecifically concentric, with successively shorter springs havingsuccessively higher spring constants. Alternatively, a smoothlyincreasing spring constant can be achieved by the bias spring 80comprising a single coil spring of smoothly varying wire thickness.

The spring constant and the preload for the bias spring 80 arepreferably higher than for the return spring 90. This ensures that mostof the driver reciprocation will be passed to the piston 50 and absorbedby the return spring 90 and not absorbed by the bias spring 80. On theother hand, the bias spring's spring constant and preload are preferablysufficiently low, and its initial length sufficiently high, to enablethe bias spring 80 to absorb the entire reciprocation stroke of thedriver 72 in a situation where the piston 50 is jammed in its fullyretracted position. Such a situation can occur if a clog in the outletline 14 totally prevents the liquid from exiting the outlet valve 44.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have elements that do not differ fromthe literal language of the claims, or if they include equivalentstructural elements with insubstantial differences from the literallanguage of the claims.

1. A pump apparatus comprising: a housing located on an axis and havinga chamber, an inlet valve and an outlet valve; a piston driverconfigured to axially reciprocate; a piston configured to reciprocate inthe chamber to draw liquid into the chamber through the inlet valveduring an intake stroke and to discharge the liquid out of the chamberthrough the outlet valve during a delivery stroke; a spring that axiallybiases the piston to a base position relative to the driver, so that thedriver, when reciprocating, will drive the piston to axiallyreciprocate; and a preload structure that preloads the spring to enablepressure of the liquid in the chamber to displace the piston away fromthe base position against the spring bias after the pressure exceeds athreshold level.
 2. The apparatus of claim 1 wherein the spring has aspring constant that increases with increasing compression of thespring.
 3. The apparatus of claim 1 wherein the spring is configured torender the volume of liquid delivered during each delivery strokeinversely related to output pressure of the pump.
 4. The apparatus ofclaim 3 wherein the spring is configured to render the volume of liquiddelivered during each delivery stroke inversely proportional to theoutput pressure.
 5. The apparatus of claim 1 wherein the preload ismanually adjustable.
 6. The apparatus of claim 5 wherein manualadjustment of the preload adjusts the depth of the piston in thechamber.
 7. The apparatus of claim 1 wherein the spring is a biasspring, and the apparatus further comprises a return spring having alower spring constant than the bias spring urging the piston out of thechamber.
 8. The apparatus of claim 1 wherein the preload structureincludes a protrusion on the piston that is located in the driver and isaxially urged by the spring against a stop surface of the driver.
 9. Theapparatus of claim 8 wherein the stop surface is between the rodprotrusion and the spring.
 10. The apparatus of claim 1 wherein thespring is configured to absorb the entire reciprocation stroke of thedriver in a case where liquid is blocked from exiting the outlet valvepiston while the driver continues to reciprocate.
 11. The apparatus ofclaim 1 further comprising an inlet hose connected to a water source.12. A pump apparatus comprising: a housing located on an axis and havinga chamber, an inlet valve and an outlet valve; a piston driverconfigured to axially reciprocate; a piston configured to reciprocate inthe chamber to draw liquid into the chamber through the inlet valveduring an intake stroke and to discharge the liquid out of the chamberthrough the outlet valve during a delivery stroke; and a spring thataxially biases the piston to a base position relative to the driver, sothat the driver, when reciprocating, will drive the piston to axiallyreciprocate while enabling pressure of the liquid in the chamber todisplace the piston away from the base position against the spring bias,the spring having a spring constant that increases with increasingcompression of the spring.
 13. The apparatus of claim 12 wherein thespring includes coil springs with different spring constants.
 14. A pumpapparatus comprising: a housing located on an axis and having a chamber,an inlet valve and an outlet valve; a piston driver configured toaxially reciprocate; a piston configured to reciprocate in the chamberto draw liquid into the chamber through the inlet valve during an intakestroke and to discharge the liquid out of the chamber through the outletvalve during a delivery stroke; a spring that axially biases the pistonto a base position relative to the driver, so that the driver, whenreciprocating, will drive the piston to axially reciprocate whileenabling pressure of the liquid in the chamber to displace the pistonaway from the base position against the spring bias; and an inlet hoseconnected to the inlet valve and configured to be connected to a liquidsource.
 15. The apparatus of claim 14 wherein the inlet hose has athreaded end configured to be screwed onto a water faucet.
 16. A pumpapparatus comprising: a housing located on an axis and having a chamber,an inlet valve and an outlet valve; a piston driver configured toaxially reciprocate; a piston configured to reciprocate in the chamberto draw liquid into the chamber through the inlet valve during an intakestroke and to discharge the liquid out of the chamber through the outletvalve during a delivery stroke; and a spring that axially biases thepiston to a base position relative to the driver, so that the driver,when reciprocating, will drive the piston to axially reciprocate whileenabling pressure of the liquid in the chamber to displace the pistonaway from the base position against the spring bias, the spring beingconfigured render the volume of liquid delivered during each deliverystroke inversely related to output pressure of the pump.
 17. Theapparatus of claim 16 wherein the delivery stroke volume isapproximately inversely proportional to output pressure of the pump. 18.A pump apparatus comprising: a housing located on an axis and having achamber, an inlet valve and an outlet valve; a piston driver configuredto axially reciprocate; a piston configured to reciprocate in thechamber to draw liquid into the chamber through the inlet valve duringan intake stroke and to discharge the liquid out of the chamber throughthe outlet valve during a delivery stroke; and a spring that axiallybiases the piston to a base position relative to the driver, so that thedriver, when reciprocating, will drive the piston to axially reciprocatewhile enabling pressure of the liquid in the chamber to displace thepiston away from the base position against the spring bias, the springbeing configured to absorb the entire reciprocation stroke of the driverin a situation where the liquid is blocked from exiting the outlet valvewhile the driver continues to reciprocate.